Free space optical interconnect systems have long been proposed to deliver fast, highly parallel data transfer. These systems have the potential to obviate limitations of electrical interconnects, which are not capable of supporting data throughputs beyond a capacity of several hundred Gb/s, and to increase the capacity up to the Terabit/s range. Thus free space interconnect systems are promising and attractive alternatives for various telecommunication and computing applications.
However, the most important challenge preventing the current acceptance of free space interconnect systems is alignment. Two issues are of concern: the precision to which it is possible to align the system, and the precision to which it is necessary to maintain this alignment during operation. For practical applications it is necessary to establish and maintain alignment of circuit boards carrying transmitters and receivers, which may comprise an array of pixels, to within 10""s of microns over a distance of meters. Such a system requires extremely expensive highly precision optomechanics, and to date has been implemented only in a controlled laboratory environment. In real product usage, when vibrations, temperature fluctuations and temperature gradients are encountered, the optical links move out of alignment and data is not correctly transferred.
Therefore, the goal of providing some alignment tolerance for optical links is to ensure the correct operation of all of the pixels on each array at the highest possible speed. Correct operation is defined as the correct reception of a logic 1 or logic 0 signal. Once the laser power, the receiver sensitivity and the detector area have been defined, the probability of correct reception of the logic bits is mainly a function of optical beam misalignment. Misalignment mechanisms can be due solely to mechanical movements, but in practice, optical effects can also contribute. Six degrees of freedom of the mechanical movements: translation in x, y, and z (xcex94x, xcex94y, xcex94z) and rotation about the x, y, and z axes (xcex8x, xcex8y, xcex8z), where x and y axes define the plane of an optical module in its nominal alignment position, with z axis being perpendicular to this plane, result in a number of optical effects. These include an image shift (xcex94x, xcex94y), image rotation (xcex8z), defocus (xcex94z) and image tilt (xcex8x, xcex8y) Image shift and rotation are basically lateral translation effects, and defocus and image tilt introduce defocus effects. Contributors to the overall lateral misalignment effects include:
mechanical misalignment in x and y;
mechanical rotation about the z axis;
mismatches in focal lengths;
wavelength shifts and laser mode-hops caused by temperature fluctuations and resulting in beam deflections introduced by diffractive elements;
distortions of the image of an array of sources by the interconnect lens system, and
telecentricity, when defocus, in addition to increasing spot size, introduces lateral misalignments in nontelecentric systems.
Contributors to the overall defocus effects include:
source array tilt;
image tilt;
curvature of the plane of best focus;
mechanical tilt about x and y axes;
misalignment along z axis.
Numerous attempts have been made to increase alignment tolerance for optical interconnect systems which may be categorized as passive, active, or dynamic strategies.
However, passive alignment of dense, high speed free space optical interconnects for distances of more than 1 cm require mechanical support structures that are too expensive, difficult to align, and insufficiently stable for commercial applications, see, e.g., xe2x80x9cOptoelectronic ATM switch employing hybrid silicon (MOS/GaAs) FET-SEEDSxe2x80x9d, A. L. Lentine et al., SPIE Proceeding, vol. 2692, pages 110-108, 1996; and xe2x80x9cOptical bus implementation system using Selfoc lensesxe2x80x9d, K. Namanaka, Optics Letters, Vol. 16, No. 16, pp. 1222-1224, August, 1991. Passive alignment is done before any devices are powered up. This alignment technique is used in almost all electrical connectors, and most optical fiber connectors are passive. Recently, solder bump techniques have been applied to certain free space optical interconnect components, and preliminary reports indicate the potential for submicron alignment in all 6 degrees of freedom over a scale of up to 1 cm, J. W. Parker xe2x80x9cOptical Interconnection for Advanced Processor Systems: A Review of the ESPRIT II OLIVES Programxe2x80x9d, L. Lightwave Technology 9 (12), 1764-1773, 1991.
Active alignment requires some feedback about the quality of the alignment. Usually the feedback is achieved by illuminating the system and monitoring the alignment either visually or by measuring a photocurrent in the detectors. Real-time active alignment is necessary if the alignment tolerances are tight or the system stability is poor so that the system will not remain aligned for a reasonable length of time. In this case, the feedback and alignment actuators must be integrated into the system to ensure permanent alignment. For example, CANON manufacturer uses image recognition and active beam-steering using a liquid filled variable angle prism in a single channel 155 Mb/s link product, which currently costs $100K. The product uses built in viewing cameras and optical pattern recognition techniques to define the system alignment, the complexity and cost of such a system clearly limiting widespread application. Alternatively, NTT has a system using actively controlled variable angle liquid filled prisms for board to board parallel free space optical interconnect, see. e.g. xe2x80x9cOptical beam direction compensating system for board-to-board free space optical interconnection in high-capacity ATM switchxe2x80x9d, K. Hirabayashi et al., Journal of Lightwave Technology, Vol. 15, No. 5, May 1997. Cost, size, environmental ruggedness and reliability of these systems remain concerns.
Additionally, to develop both a marketable and reliable system, devices have to be packaged in a useful and reliable manner. For large systems including cumbersome and bulky mechanical parts providing alignment, this could involve an significant amount of physical space just to house all the individual components.
Recently, a proposal for avoiding high precision mechanics in free space interconnect systems by use of redundant detectors has been put forward by F. A. P. Tooley in IEEE Journal of Selected Topics in Quantum Electronics April 1996, vol. 2, No. 1, pp. 3-13 and in Digest, IEEE Summer Topical Meetings, Aug. 5-9, 1996, p. 55-56. This system increases tolerance to misalignment by providing an array of detectors in place of a single detector and electrically re-routing the misaligned optical data to the correct channel, or, alternatively, by replicating the signal a number of times. The overhead associated with increasing the alignment tolerance requires a control and router circuit, which adds electrical power consumption.
Therefore a need exists for development of alternative structures for free space optical interconnect systems which would avoid high precision mechanics, while providing precise alignment combined with simple design, reliability, low power consumption and compact packaging.
Thus, the present invention seeks to provide an optical interconnect system and method which avoid or reduce the above-mentioned problems.
Therefore, according to one aspect of the present invention there is provided a free space optical interconnect system comprising:
a transmitter and a receiver, at least one of the transmitter and the receiver comprising a plurality of elements arranged into clusters, the number of clusters being redundant and the number of elements in each cluster being sufficient to accommodate the number of data channels to be transmitted;
means for identifying a misalignment between the transmitter and the receiver; and
means for re-routing data from the cluster which is misaligned to a redundant cluster providing data transmission through the system, the re-routing being performed in response to a signal generated by the means for identifying the misalignment.
Conveniently, the means for identifying the misalignment comprises means for providing feedback between the transmitter and the receiver regarding the misalignment.
In the first embodiment of the invention, the number of elements in each cluster is equal to the number of data channels to be transmitted. Alternatively the number of elements in a cluster may be more than the number of the transmitted data channels, with the means for re-routing data between the clusters further comprising means for re-routing data between the elements within a cluster. It is also possible to arrange that the number of elements in each cluster is less than the number of data channels to be transmitted, e.g. by using transmitter elements capable of transmitting more than one data channel (multi-wavelength lasers). The number of elements in different cluster may be equal or different, depending on the system requirements.
The elements of the transmitter and/or the receiver may be arranged into clusters, the clusters preferably being arranged into a one-dimensional or two-dimensional array, or any other pattern providing the required optical transmission or collection. The elements within clusters of the transmitter and/or receiver may also be arranged into a pre-determined pattern, and individual elements may or may not be shared by different clusters. The system may comprise one transmitter and one receiver only to provide a uni-directional interconnection. Alternatively, the system comprises two modules, each comprising one transmitter and one receiver, thus providing for a bi-directional data transmission and receiving of data.
Preferably, the system is implemented with optical elements, such as bulk optics (lenses, prisms, mirrors, splitters, et al.), binary optics (fanout gratings, diffractive lenses, et al.), holographic elements, and integrated optics.
Preferably, the elements of the transmitter are optical emitters or optical modulators. The emitters may be vertical cavity surface emitting lasers (VCSEL), light emitting diodes (LED) and edge emitting laser diodes or other known devices. The modulators may be modulators based on magneto-optic effect, modulators including liquid crystal devices, ferroelectric modulators, e.g. lead lanthanum zirconate titanate (PLZT) modulator, modulators including piezo-electric crystals, modulators including deformable mirrors, electro-optical semiconductor hetero-structure modulators, optical cavity modulators, or other known modulators.
The receiver of the optical interconnect system comprises at least one detector, preferably from PIN detector, metal-semiconductor-metal detector, avalanche photodiode, or other known detectors.
To identify misalignments of the system, the system includes identifying means, e.g. detectors for monitoring lateral and vertical misalignments, detectors for monitoring tilt misalignments, at least one dedicated alignment laser and at least one dedicated detector, and means for monitoring a signal level at the dedicated detector or detectors.
To provide feedback between the transmitter and the receiver regarding misalignments of the system, the system includes means providing a stable feedback mechanism which may be selected from optical fiber, LED, electrical cable, electrical backplane, or other convenient means.
When misalignments of the system occur, each cluster accommodates for misalignments within a predetermined spatial and angular deviation, the data being re-routed between clusters when the misalignment is beyond the deviation. Preferably, means for re-routing of data provide cycling through the clusters of at least one of the transmitter and the receiver according to a predetermined orthogonal pattern which ensures alignment of the system. Alternatively, re-routing of data may be done by cycling through the clusters at different rates or any other method to provide alignment of the system. In the case of a system redundancy both of lasers and of and of detectors, preferably the lasers compensate for a gross misalignment, and the detectors simultaneously make additional fine compensation of misalignment. Preferably, the transmitter and/or receiver, or, alternatively, the whole system described are integrated within a package or several packages, thus providing compactness and efficient use of space.
According to another aspect of the invention there is provided a method of compensating misalignments in a free space optical interconnect system comprising a transmitter and a receiver, at least one of the transmitter and the receiver comprising a plurality of elements whose number is redundant, the elements of at least one of the transmitter and the receiver being arranged into clusters, the number of clusters being redundant and the number of elements in each cluster being sufficient to accommodate the number of data channels to be transmitted, the method comprising the steps of:
identifying a misalignment between the transmitter and the receiver; and
re-routing data from the cluster which is misaligned to a redundant cluster providing data transmission through the system, the re-routing being performed in response to a signal generated at the step of identifying the misalignment.
Conveniently, the step of identifying the misalignment further comprises sending a feedback signal between the transmitter and the receiver regarding the misalignment. Additionally, the method may further include a step of arranging that the number of elements in each cluster is equal to the number of data channels to be transmitted. Alternatively, it may be arranged that the number of elements in each cluster is not equal to the number of the transmitted channels, e.g. being more than the number of channels. In this situation, the step of re-routing data between the clusters may further comprise re-routing of data between the elements within a cluster.
Beneficially, the method provides a continuous misalignment compensation of the system within a predetermined angular and space deviation, the identifying of misalignments being made by monitoring a signal level at the receiver. Preferably, re-routing of data is performed by cycling through the clusters according to a predetermined orthogonal pattern or by cycling through the clusters at different rates ensuring alignment of the system, and the elements of the transmitter and/or receiver may or may not be shared by different clusters.
According to yet another aspect of the invention there is provided a method of compensating misalignments in a bi-directional free space optical interconnect system comprising a first module and a second module, each module having a transmitter and a receiver, at least one of the transmitter and the receiver at each module comprising a plurality of elements arranged into clusters, the number of clusters being redundant and the number of elements in each cluster being sufficient to accommodate the number of data channels to be transmitted, the method comprising the steps of:
(a) defining an orthogonal sequence of pairs of clusters, each pair comprising one cluster from each module;
(b) choosing a first pair from the sequence;
(c) re-routing data to the selected pair of clusters;
(d) monitoring corresponding signal levels of the data at the receivers;
(e) comparing signal levels at the receivers with predetermined threshold values;
(f) when the signal level at least at one of the receivers is below the threshold value, re-routing the data to the next pair of clusters from the sequence and repeating the steps (d), (e) and (f).
According to yet another aspect of the invention there is provided a module for a free space optical interconnect system, comprising:
at least one of a transmitter and a receiver, at least one of the transmitter and the receiver comprising a plurality of elements arranged into clusters, the number of clusters being redundant and the number of elements in each cluster being sufficient to accommodate the number of data channels to be transmitted;
means for re-routing data from the cluster which is misaligned to a redundant cluster in response to feedback identifying a misalignment of the module.
Conveniently, the number of elements in each cluster is equal to the number of data channels to be transmitted. Alternatively, the number of elements in each cluster may be more than the number of data channels to be transmitted, with the means for re-routing data between the clusters further comprising means for re-routing data between the elements within a cluster. It is also possible to arrange that the number of the elements within the cluster is less that the number of the data channels to be transmitted, e.g. by using multi-wavelength lasers. The number of elements in different clusters may be equal or different depending on the module requirements.
Conveniently, the module further comprises means for identifying a misalignment of the module in the system, which may include detectors for monitoring lateral and vertical misalignments, detectors for monitoring tilt misalignments, a dedicated alignment laser and a dedicated detector, or means for monitoring a signal level at the receiver.
Preferably, the clusters of the module are arranged in a one-dimensional or two-dimensional array, or any other pattern providing a required light transmission or collection. The module may include one transmitter only or one receiver only for corresponding uni-directional transmittance or reception of data. Alternatively, the module may include both a transmitter and a receiver for corresponding transmitting and receiving of data in a bi-directional optical interconnect system. The elements of the transmitter and/or receiver may or may not be shared by different clusters, the elements of the transmitter being preferably optical emitters or optical modulators. Preferably, the module described above is integrated within a package.
Free space interconnect systems formed using the techniques described above are much more tolerant to misalignments between circuit packs compared to electrical connectors or other existing free space optical interconnect systems. The use of redundant elements of the transmitter, or redundant clusters of elements in the transmitter or receiver modules obviates the need of packaging which requires precise alignment and which is often expensive and bulky. The interconnect systems based on the present invention have simpler mechanical design, have no moving parts and may be implemented with lower cost mechanics. As a result, they can be manufactured more readily and at much lower cost, and providing higher reliability at the same time.